The threading of pull-cords through conduits in buildings for the purpose of pulling electrical wires and communications cables through the conduits is time consuming and often difficult due to the fact that the conduits may be lengthy and have multiple bends.
The most common method for threading electrical wires or pull-cords through conduits makes use of a long spring steel wire, often referred to in the trade as a fishwire or fishtape, that is manually pushed through a conduit from one end to the other. An electrical wire or communications cable or a plurality thereof may then be secured to the free end of the fishtape that is then pulled back through the conduit in order to pull the electrical wire or communications cable through the conduit. This method is often time consuming and difficult to carry out as the fishwire generally has to be forced around a series of bends of the conduit, sometimes with great resistance, and sometimes without success. The one fishwire may then have to be retrieved from the opposite end of the conduit with a second fish wire.
A wide variety of different devices has been proposed in the prior art that utilise, in most cases, an elevated pressure, typically that of compressed air at high pressure in canisters, but in other cases an elevated or sub-atmospheric pressure created using an electrical blower, most typically, a vacuum cleaner.
Thus, for example, British patent GB 1910 10705 describes the introduction of a pull-cord that has a reel on which the cord is wound and a source of compressed air to blow a dart with an expendable flange through the conduit with one end of the cord attached to its head. This arrangement has the disadvantage that high pressure can build up behind the dart especially if the movement of the dart along the conduit takes place irregularly according to obstructions typically in the form of foreign matter encountered and also bends in the conduit that have to be negotiated; and the general effect can be that dirt, water and other debris can be expelled from the open end of the conduit with some force and velocity thereby creating a safety hazard to property and persons who may be in the vicinity of the open end of the conduit. Also, the unwinding of a cord from a reel is considered by applicant to be inappropriate to the effective implementation of such a method.
Still further, the operator is generally not able to observe the arrival of the pull-cord at its destination, resulting in an undesirable lack of control especially on noisy building sites where communication is often a problem.
A similar high pressure system is proposed in U.S. Pat. No. 4,043,537 to Russo which accordingly has the disadvantages associated with high pressure that are indicated above.
On the other hand, U.S. Pat. No. 5,246,207 to Horii et al; U.S. Pat. No. 5,374,034 to Flores et al; and U.S. Pat. No. 5,730,424 to Flores Snr all propose systems in which there is no plug or dart to carry an end of the cord or line that is simply blown through the conduit utilising elevated fluid pressure that once more has the disadvantages indicated above.
U.S. Pat. No. 3,793,732 to Hamrick discloses a similar arrangement that utilises either the elevated pressure or suction developed by a vacuum cleaner to achieve the same objective. Whilst the positive or negative pressure developed by a vacuum cleaner is unlikely to be sufficiently high to create a danger, the air pressure differential generated is also considered by applicant to be insufficiently high to be effective for the purpose at hand. Once more a reel of cord is employed with attendant disadvantages.